The present invention is concerned with mobile telecommunication systems comprising a number of base stations which can communicate with mobile stations. FIG. 1 shows a base station BTS in communication with three mobile stations MST, MS2 and MS3. The communication from a mobile station MSi to the base station BTS is done by means of an up-link UL and the communication from the base station BTS to a mobile station MSi is done by means of a down-link DL.
The present invention is also concerned with telecommunication systems wherein different user signals are separated both in time domain and in code domain. An example of such system is the so called UMTS TDD system or W-CDMA TDD system in which the time domain is represented by the TDD-system component and the code domain by CDMA-system component.
More particularly, in time-domain, transmission is for example organised based on radio frames constituted of a number N (for example N=15) of timeslots. The same frequency is used for both the up-link (Mobile Station to Base Station) and the down-link (Base Station to Mobile Station). Furthermore, a time-separation is used to differentiate the down-link and the up-link such that a subset of the N available timeslots per frame is exclusively allocated for down-link transmission and the remaining ones for up-link transmission. In a frame, at least one timeslot is always allocated for each down-link and up-link.
In such a system, different user's signals can be transmitted in separate timeslots, e.g. N different down-link timeslots are allocated to N different down-link user signals. This is the time-domain of the system. Furthermore, several users signals can also be transmitted within one timeslot by using different spreading codes. This is the code-domain mode of the system.
In such a system, all base stations in an area operate synchronously and generally share the same up-link/down-link timeslot configurations.
In both up-link and down-link, user's data is transmitted in a timeslot arranged in a burst B comprising, as illustrated in FIG. 2, a first data field D1, a general midamble field M and a second data field D2. A midamble is a complex-valued chip sequence and is used by a receive (the base station BTS in the up-link or a mobile station in the down-link) for channel estimation which is needed for the retrieval of the user's signals.
In the up-link, each mobile station MSi sends a different midamble m(i), as the base station BTS needs an individual channel estimation for each mobile station transmitting in a particular timeslot.
Note that when a midamble is not explicitly assigned to a mobile station, a default fixed-allocation rule between its assigned spreading code and a particular midamble is used.
In the down-link shown in FIG. 2, generally just one midamble m(i) is used by the base station BTS for all user's signals within a particular timeslot. The reason is that in the down-link, all users experience just one down-link channel to estimate, e.g. from the base station BTS to itself and ignore those of the other users transmitting in the same timeslot. But in some situation, when more than one channel estimation is needed, more that one midamble can be used by a base station BTS. In this cases, the midamble M results in the summation of all these midambles.
A guard period G can be provided to ensure proper separation in time of consecutive timeslots. Also, signalling bits S can be provided.
In the up-link UL, data of a mobile station MSi is spread to the chip rate by a complex valued spreading code ai (or the spreading codes) which is (are) affected to this mobile station MSi by the system.
In the down-link DL, each data di intended for a mobile station MSi is spread to the chip rate by a corresponding spreading code ai (in 11 to 1k on FIG. 2), the results of all these spreading operations being summed (in 20) to form the data D1 and D2 contained in the burst.
A problem occurs when an advanced detection algorithm such as Joint Detection is used for the retrieval of the user's signals at the receiver side. With such an algorithm implemented, data bits from all users transmitting in a timeslot are simultaneously decoded and decided at receiver-side. For optimal performance of the algorithm, the receiver needs to know several parameters, especially spreading codes and channel profiles of all users which are present in a particular timeslot.
Generally, when such an algorithm is implemented at a base station-side, the base station can have a knowledge of the allocated spreading codes because the radio access network to which it belongs controls their usage.
But, the situation is quite different, when the considered algorithm is implemented at the mobile station in the down-link. A mobile station doesn't generally know the other spreading codes which are allocated to the other user's signals simultaneously present in the same timeslot. This fact seriously impacts the implementation of the algorithm, such the Joint-Detection, at mobile station-side.
One first possibility to overcome this problem is to perform a so-called “Blind spreading-code detection” where it is tested for, for instance by despreading and thresh-holding at mobile station-side, if some or all possible spreading codes are used in a particular timeslot.
A second possibility consists in communicating to each mobile station all spreading codes which are currently used by all user's signals present in one particular timeslot. This solution is practicable only if this signalling can be done fast and with only marginal delay. This last constraint especially makes an explicit signalling by multiplexing signalling bits together with the data bits contained in the data fields of a burst not easy to implement.